1. Field of the Invention
The present invention relates to a magneto-optical information reproducing apparatus which reproduces information magnetically recorded on a recording medium by utilizing a magneto-optical effect, and more particularly to an optical element which is to be used in an optical head of the apparatus and which is simple in structure and yet assures a high performance of the apparatus and significantly contributes to reduction of size, weight and cost of the apparatus.
2. Related Background Art
An optical memory which records and reproduces information by a semiconductor laser beam has been recently researched and development vigorously for practical use as a high recording density memory. A read-only optical disk and a DRAW type optical disk which have been commercialized as compact disks as well as an erasable and rewritable magneto-optical disk are considered promising. In the magneto-optical disk, information is magnetically recorded by utilizing a local temperature rise of a magnetic thin film by irradiation of a laser beam spot, and the information is reproduced by a magneto-optical effect (particularly a Kerr effect). The Kerr effect is a phenomenon in which polarization plane is rotated when light is reflected by a magnetic recording medium.
A basic configuration of a prior art magneto-optical disk apparatus is shown in FIG. 1. In FIG. 1, numeral 1 denotes a semiconductor laser, numeral 2 denotes a collimator lens, numerals 11 and 12 denote half-mirrors, numeral 4 denotes an objective lens, numeral 6 denotes a magneto-optical recording medium, numerals 7.sub.1 and 7.sub.2 denote analyzers, numeral 8 denotes a focusing lens, and numerals 9.sub.1 and 9.sub.2 denote photo-detectors. A P-polarized light is parallel to the plane of the drawing and an S-polarized light is normal to the plane of the drawing.
The reproduction of the magneto-optical information in the above apparatus is explained below. A light beam emitted from the semiconductor laser 1 as a linearly polarized P-polarized light is collimated by the collimator lens 2 and passes through the half-mirror 11. In the half-mirror 11, .vertline.t.sub.OP .vertline..sup.2 =.vertline.t.sub.OS .vertline..sup.2 =0.5 where t.sub.op is an amplitude transmittance of a P-polarization component and t.sub.OS is an amplitude transmission of an S-polarization component. The light beam is focused on the magneto-optical recording medium 6 by the objective lens 4 as a fine spot. When a magnetic domain (pit) is preformed on the medium 6, a reflected light from the medium 6 has a polarization plane thereof rotated by .+-..theta..sub.K by the Kerr effect depending on whether the direction of magnetization at the area of the spot irradiation is upward or downward. The following relationship exists between a P-polarization component R and an S-polarization component K of an amplitude reflectance of the recording medium 6. ##EQU1##
The magneto-optically modulated reflected light is again collimated by the objective lens 4, is reflected by the half-mirror 11, converged by the focusing lens 8 and split by the half-mirror 12. The split beams pass through the analyzers 7.sub.1 and 7.sub.2 and are detected by the photodetectors 9.sub.1 and 9.sub.2 as intensity-modulated light beams. As shown in FIG. 2, an angle between an optical axis of the analyzer and the P-polarization direction is .+-..theta..sub.A on the transmission side and the reflection side, respectively, and the light beam is detected as an orthogonal projection of the amplitude to the optical axis of the analyzer.
The Kerr rotation angle .theta..sub.K is approximately 1 degree and the magneto-optical modulation component is very small. Accordingly, it is necessary to set an azimuth angle .theta..sub.A of the optical axis of the analyzer to an optimum one such that a C/N (carrier to noise ratio) of the detected signal is maximum. For example, in the prior art apparatus, the azimuth angle .theta..sub.A of the optical axis of the analyzer is set to 45 degrees so that the signal light is maximum. When the Kerr rotation angle is +.THETA..sub.K, the intensities of the lights which pass through the analyzers in the transmission side and the reflection side and are directed to the photodetectors, respectively, are given by ##EQU2## where I.sub.O is the intensity of incident light to the recording medium. Since .theta..sub.K is approximately one degree, .vertline.R.vertline..sup.2 &gt;&gt;.vertline.K.vertline..sup.2 is met. Accordingly, the formula (2) is expressed as ##EQU3##
A second term in the parentheses of the formula (3) represents the magneto-optical modulation component, and a first term represents a non-modulation component. Those lights are photo-electrically converted by the photo-detectors 9.sub.1 and 9.sub.2 and they are differentially detected by a differential circuit (not shown) to produce a magneto-optical signal.
However, when noise superimposed on the detected signal is taken into account, it is not always true that a maximum C/N ratio is attained by setting .theta..sub.A to 45 degrees.
On the other hand, in order to improve the C/N ratio of the reproduced signal, Japanese Laid-Open Patent Application No. 57-200958 proposes a magneto-optical information reproducing apparatus which uses a polarization beam splitter in place of the half-mirror 11 of FIG. 1.
Japanese Laid-Open Patent Application No. 58-128037 discloses an apparatus which uses a one-half wavelength plate and a polarization beam splitter (PBS) in place of the half-mirror 12 and the analyzers 7.sub.1 and 7.sub.2. This is explained with reference to FIG. 3.
In FIG. 3, a light beam emitted from a light source (laser diode) 27 passes through a beam splitter 24, a mirror 25 and an objective lens 26 and is directed to a recording medium 31. The light beam reflected by the medium 31 then passes through the objective lens 26 and the mirror 25 and is directed to the beam splitter 24, and it is reflected toward a focusing lens 23 by the beam splitter 24. Numeral 33 denotes a direction of polarization when the polarized light emitted from the light source 27 travels toward the focusing lens 23 without being subjected to the Kerr effect. Actually, however, when the light is reflected by the medium 31, the polarization plane is rotated by the Kerr rotation angle .+-..theta..sub.K with respect to the direction 33 and is directed to the focusing lens 23. The polarization plane is rotated by 45 degrees by the one-half wavelength plate 34 which has the optical axis thereof inclined by 22.5 degrees with respect to the direction 33 so that it is polarized in the direction 35. A portion of the light beam thus polarized is reflected by the beam splitter 22 and is detected by a servo signal detecting sensor (photo-electric converter) 28. On the other hand, the light beam which has transmitted through the beam splitter 22 is directed to the PBS 21. The light beam directed to the PBS 21 is split into two light beams which are polarized in orthogonal directions, and they are detected by magneto-optical signal sensors (photo-electric converters) 29 and 30, respectively. By differentiating the detection signals of the sensors, the information recorded on the-medium is reproduced.
However, in the apparatus shown in FIG. 3, the number of parts required is large and the assembly of those parts is not easy because the azimuth angles of the one-half wavelength plate 34 and the PBS 21 must be strictly adjusted with respect to the direction of polarization of the light beam.